In home fuel cell systems, it is desirable to carry out a DSS (Daily Start & Stop or Daily Start-up & Shut-down) operation in which the operation of the fuel cell system is carried out during daytime and is stopped during midnight so that electricity and heating expenses are reduced and carbon dioxide is effectively reduced. In the DSS operation, the operation of the fuel cell system is frequently stopped. In the conventional fuel cell system, such an operation arises a problem that electrocatalyst is degraded due to reaction gases remaining within a stack or air entering from outside during a state where the fuel cell system is not generating an electric power and hence performance of a fuel cell is deteriorated.
As a fuel cell system which was made to solve such a problem, for example, there is a fuel cell system in which, in a start-up state, a means for consuming an electric power is internally connected in a time period before an external load is connected to the system so that occurrence of an open circuit potential is prevented (see patent document 1). Also, there is a fuel cell system equipped therein with an electric discharge means for suppressing an open circuit voltage (see patent document 2). Also, there is a fuel cell system in which performance of a cathode electrode is improved by using hydrogen that leaks from an anode to a cathode (see patent document 3). Also, there is a fuel cell system in which a gas discharged from a cathode is re-circulated into the cathode to consume oxygen, thereby improving durability (see patent document 4). Also, there is a fuel cell system in which power generation is carried out in a state where gas supply is stopped to consume reaction gases remaining within a fuel cell, thereby decreasing electrode potential (see patent document 5). Also, there is a fuel cell system in which a fuel cell is activated while supplying a hydrogen-containing fuel to an anode, thereby decreasing a cathode potential (see patent document 6).    Patent document 1: Japanese Laid-Open Patent Application Publication No. Hei. 5-251101    Patent document 2: Japanese Laid-Open Patent Application Publication No. Hei. 8-222258    Patent document 3: Japanese Laid-Open Patent Application Publication No. 2000-260454    Patent document 4: Japanese Laid-Open Patent Application Publication No. 2003-115317    Patent document 5: Japanese Laid-Open Patent Application Publication No. 2004-186137    Patent document 6: Japanese Laid-Open Patent Application Publication No. 2003-536232